Cyclotron square wave rf system



Jan. 22, 1957 G. B. ROSSI 2,778,937

CYCLOTRON SQUARE WAVE RF SYSTEM Filed April 22, 1954 MAIN 5/ OSCILLATORSECONDARY OSCI LL ATOR INVENTOR. GUIDO B. ROSS! BY /M4 4% ATTORNEY.

CYCLOTRON SQUARE WAVE RF SYSTEM Guido B. Rossi, Berkeley, Calif.,assignor to the United States of America as represented by the UnitedStates Atomic Energy Commission Application April 22, 1954, Serial No.425,061

10 Claims. (Cl. 250-27) The present invention relates to a particleaccelerator and more particularly to an improved radio frequency systemfor a cyclotron.

The principles of operation of cyclotrons are well known in the art,including those of fixed frequency cyclotrons. Several problems existwhich limit the stability 'of particles being accelerated, as Well asthe final operating conditions. Most of such problems are related to thephasing between the radio frequency accelerating voltage, thelimitations imposed by design parameters, and the angular velocity ofthe particles.

It has been determined that, by replacing the sine wave radio frequencyaccelerating voltage with a square wave voltage of the same maximumvalue, improvements in accelerating conditions result from (a) anaverage dee voltage increase of and '(b) a change in time distributionof the peak accelerating voltage. Under the former condition (a) theoperation improvements resulting from higher dee voltages include anincreased ion catching phase, a decreased number of revolutions to theextraction radius,

and a decreased phase defocusing. Under the latter condition (b) thepeak accelerating voltage appears through substantially 180 degrees ofthe radio frequency cycle and additional operation improvements resultwhich cannot be gained simply by increasing the maximum value of a sinewave of acceleration voltage. Such advantages are (l) greatercompensation for variations in the angular velocity of the particles,(2) better initial bunching of ions at the start of the accelerationcycle, (3) fewer particle revolutions to the extraction energy, and (4)a possible compensation for relativistic loss of phase when such lossesare not large. Also, where dee voltage limitations exist (set by dee toground discharges), such effects are especially advantageous, since theresult is an action to increase maximum beam levels for the availablepeak values of accelerating voltage.

In spite of the foregoing desirable advantages to be gained, it is notfeasible to directly excite the high Q resonant circuits, as associatedwith the cyclotron, with a square wave. However, to achieve theadvantages of a square wave acceleration voltage, the present inventionprovides for a mixing of harmonics with a fundamental sine wave toproduce an approximation of a square wave voltage at the dees. That suchmixing of voltages is feasible to achieve the desired waveform will bereadily apparent by considering well-known principles of the electricalart. A sine wave used for excitation of conventional cyclotronaccelerating electrodes may be represented by the equation:

V=A cos Where V is the value of the instantaneous voltage, A is thevalue of the peak voltage, and 0 is the ,phase angle. It is well knownthat the Fourier series gives the equatlon "nited States Patent 0'paratus for the Acceleration of Ions.

2,778,937 Patented Jan. 22, 1957 of a square wave and so, in similarterms, the equation may be written:

V=A1 cos 0-A cos 30+A3 cos 56 +(1)" An cos (2n--1).0

It has been found that a wave containing the .first two terms of thelatter formula approximates a square wave where A2 is made substantiallyequal to 1/ 9 A1. The first term (A1 cos 0) represents the fundamentalsine wave and the second term (-A2 cos 30) represents the third harmonicof such fundamental sine wave. While not affording the maximum benefitsof a full square wave, the mixing of the fundamental and third harmonicsprovides a close approximation to extend the period of the peak voltageby a factor of two or more.

It is therefore an object of the invention to provide an improved radiofrequency system for a cyclotron.

Another object of the invention is to provide a square wave acceleratingvoltage at the does of a cyclotron.

Still another object of the invention is to provide a constant frequencycyclotron characterized by increased average dee voltage, beamintensity, and beam stability.

A further object of the invention is to provide an improved radiofrequency accelerating system for a cyclotron wherein harmonics and afundamental sine wave are mixed to produce a square Wave acceleratingvoltage.

Other objects and advantages of the invention will be apparent in thefollowing description and claims considered together with theaccompanying drawing which is a semi-schematic and sectional View of theinvention.

Referring to the drawing in detail there is illustrated a portion of acyclotron 11 which is similar to that described and claimed in U. S.Patent 1,948,384 issued to E. 0. Lawrence, February 20, 1934, for Methodand Ap- Such cyclotron 11 comprises, in general, a vacuum tank 12disposed between pole faces 13 (one of which is shown) of anelectromagnet, which is excited by windings 14, and a pair of deeelectrodes 16 and 17 disposed 'within the tank. Various circuits andother elements are necessary for a detailed description of the operationof the cyclotron; however, such elements are conventional and a detailedaccount thereof is omitted to provide a better understanding of thepresent invention.

A main oscillator 21 and a secondary oscillator 22, both of conventionaldesign, are provided to furnish radio frequency accelerating voltagesfor the cyclotron 11. The frequency of the main oscillator 21 isestablished at 'a fundamental frequency in accordance with the type ofparticles to be accelerated and the frequency of the secsynchronizingpurposes to maintain the proper frequency difierence and phaserelationship therebetween.

To suitably couple the outputs of the oscillators 21 and 22 to the deeelectrodes 16 and 17 of the cyclotron 11, there are provided a pair ofsimilar radio frequency systerns 26, only one of which will be describedin detail. Such a system 26 comprises, in general, a coaxialtransmission :line having an inner conductor 27 and an outer conductor28. At one end the system 26 is connected with the inner conductor 27tied to the dee electrode 16 through an opening 29 in the tank 12 andthe outer condoctor 28 tied to the tank about such opening in avacuum-tight manner, as by welding. The other endo'f the system 26 isextended, externally of the tank 12, for a distance substantially equalto an electrical length of an odd number of quater-wavelengths at thefrequency of the main oscillator 21. The outer conductor 28 at suchextremity is sealed to maintain the vacuum-tight condition of thecyclotron 11 while the inner conductor 27 is terminated short of theouter conductor seal and is suitably supported by a disc 31 ofconducting material.

A stub 36 is included in the system 26 by electrically connecting alength of conductor 37 at one end to the inner conductor 27 andextending the same transversely through an opening 38 in the outerconductor 28. A conducting sheath 39 having a length greater than theextended portion of the stub conductor 37 is electrically connectedabout the opening 38 as a vacuum seal and as the outer conductor of thecoaxial stub 36. The electrical length of the stub 36 is selectedtoprovide a capacitive reactance (one quarter-wavelength or less) at thefrequency of the secondary oscillator 22 and the point of connection tothe inner conductor 27 is selected along the inner conductor from thedee electrode 16 so that the capacitance and a portion of the inductanceof the system form a resonant circuit at such frequency.

The two radio frequency systems 26 extend in a substantially parallelmanner from the cyclotron tank 12 and are suitably excited by the mainand secondary oscillators 21 and 22. To provide such excitation anoutput terminal 41 of the main oscillator 21 is connected by a radiofrequency lead 42 to a first coupling loop 43 extending through theouter conductor 28 of the systom 26 which is connected to the deeelectrode 16 be tween the dee electrode and the stub 36. Also an outputterminal 46 of the secondary oscillator 22 is connected by a lead 47 toa second coupling loop 48 extending through the outer conductor 28between the stub 36 and the disc 31. Similarly, a second output terminal51 of the main oscillator 21 and a second output terminal 52 of thesecondary oscillator 22 are connected to coupling loops 53 and 54 of theother system 26 by leads 56 and 57, respectively.

It is to be noted that, while the secondary oscillator 22 has beenspecified in the foregoing as an essential element as an aid tounderstanding the invention, such secondary oscillator and auxiliaryelements are not required where the inherently occurring third harmonicfrequency component of the fundamental frequency exists in the systems26 with a sufficiently strong peak value. In such instance the presenceof the stubs 36 provide a peaking effect at the third harmonic frequencyand reflect a voltage at such frequency having a value which mixes withthe voltage of the fundamental frequency at the dee electrodes 16 and 17to provide the desired waveform. Thus it is only where the value of thevoltage at the third harmonic frequency is to be controllable that thesecondary oscillator 22 is required.

Now with the conditions for operation of the cyclotron 11 established,the main oscillator 21 and the secondary oscillator 22 are energized torespectively establish output voltages at a fundamental frequency and atthree times the fundamental frequency with the latter having a peakvalue sufiicient to impress a voltage at the dee electrodes 16 and 17which has a peak value one-ninth that of the former. Such voltagesseparately excite the two radio frequency systems 26 with voltages 180electrical degrees apart for both the fundamental and third harmonicfrequencies.

Considering the fundamental frequency excitation first, it will be notedthat the electrical length of the systems 26 and does 16, 17 isestablished to be an odd number of quarter-wavelengths and that theextremity away from the dee electrodes 16 and 17 is short circuited bythe disc 31. The result of such conditions is that a voltage maxima isimpressed at the dee electrodes 16 and 17, respectively.

Now, considering the voltage having a frequency three times that of thefundamental frequency, it will be noted that a resonant circuit at thethird harmonic frequency has been provided across the coaxial system 26(the stubs 36) to maximize the voltage at such harmonic frequency in thesystems 26. With voltages at both frequencies impressed at the deeelectrodes 16 and 17 the resultant voltage between such electrodes isthe sum of the two. As has been stated previously, the two oscillators21 and 22 are interconnected so that the proper frequency and phaserelationship is maintained to provide an approximation of a squarewaveform of voltage between the dee electrodes 16 and 17 with theattendant advantages.

While the salient features of the present invention have been describedin detail with respect to one embodiment it will be apparent thatnumerous modifications may be made within the spirit and scope of theinvention and it is therefore not desired to limit the inven tion to theexact details shown except insofar as they may be defined in thefollowing claims.

What is claimed is:

1. In a radio-frequency system for a cyclotron having a pair of deeelectrodes, the combination comprising an oscillator developing avoltage with a fundamental frequency component and a third harmonicfrequency component having a peak value substantially one-ninth that ofthe fundamental component, two quarter-wavelength transmission linesystems respectively connected to said dee electrodes, and couplingmeans connected between said oscillator and said systems, each of saidsystems including means resonant at the third harmonic frequencydisposed to maximize the third harmonic component at said dee electrodesand provide a substantially square wave of voltage.

2. In a radio-frequency system for a cyclotron having a pair of deeelectrodes, the combination compris ing an oscillator developing avoltage with a fundamental frequency component and a third harmonicfrequency component having a peak value substantially one-ninth that ofthe fundamental component, two quarter wavelength coaxial transmissionline systems respectively connected to said dee electrodes, and couplingmeans connected between said oscillator and said systems, each of saidsystems including an open-ended coaxial stub having an electrical lengthestablishing a resonant circuit at the third harmonic frequency disposedto maximize the third harmonic component at said dee electrodes andprovide a substantially square wave of voltage is impressed between saiddee electrodes.

3. In a radio-frequency system for a cyclotron having a pair of deeelectrodes, the combination comprising a first oscillator developing avoltage at a fundamental frequency, a second oscillator developing avoltage at three times the fundamental frequency, two transmission linesystems singly connected to said dee electrodes, and means connectedbetween said oscillators and systems for coupling the voltages of eachoscillator to each of said systems, each of said systems including meansfor combining both voltages at said dee electrodes to provide asubstantially square wave accelerating voltage.

4. In a radio-frequency system for a cyclotron having a first and asecond dee electrode, the combination comprising a first oscillatordeveloping a voltage at a fundamental frequency, a second oscillatordeveloping a voltage at three times the fundamental frequency, twoshort-circuited transmission line systems singly connected to said deeelectrodes, and means connected between said oscillators and systems forcoupling the voltages of each oscillator to each system, each of saidsystems being substantially an odd number of electricalquartenwavelengths at said fundamental frequency in length and includingmeans providing a resonant circuit at the frequency of said secondoscillator for impressing the voltage of such oscillators at said deeelectrodes.

5. In a radio-frequency system for a cyclotron hav ing a pair of deeelectrodes, the combination comp-rising a first oscillator developing avoltage at a fundamental frequency, a Second oscillator developing avoltage at three times said fundamental frequency, two coaxialtransmission line systems singly connected to said dee electrodes andbeing short-circuited at a point located an odd number ofquarter-wavelengths at said fundamental frequency from such connection,a coaxial stub connected to each of said systems to provide a resonantcircuit at the frequency of said second oscillator, and means connectedbetween said oscillators and each of said systems for coupling thevoltages of the former to the latter.

6. In a radio-frequency system for a cyclotron having a pair of deeelectrodes, the combination comprising a first oscillator developing avoltage at a fundamental frequency, a second oscillator developing avoltage at three times said fundamental frequency, two coaxialtransmission line systems having outer and inner conductors singlyconnected to said dee electrodes, a connecting conductor disposedbetween outer and inner conductors of said systems at a point located anodd number of quarter-Wavelengths from said dee electrodes at thefrequency of said first oscillator, an open-ended coaxial stub havinginner and outer conductors respectively connected to the inner and outerconductors of each of said systems, said stub being less than onequarter-Wavelength at the frequency of said second oscillator and beingconnected to said system to provide a resonant circuit at the frequencyof said second oscillator, and coupling means connected between saidoscillators and each of said systems.

7. The combination of claim 3 wherein the voltage of said secondoscillator is characterized as having a peak value established toprovide a voltage of substantially one-ninth of the peak value of thevoltage of said first oscillator at said dee electrodes.

8. In a cyclotron, the combination comprising a pair of similar firsttransmission lines having an outer co-ndoctor and a coaxial innerconductor, said first transmission lines being resonant at thefundamental frequency of the cyclotron and having an open extremity, adee electrode joined with said inner conductor at said open extremity ofeach of said first transmission lines, a stub transmission line securedto each of said first transmission lines and electrically connectedtherewith, said stub transmission line being resonant at the thirdharmonic of said fundamental frequency and being disposed at such pointon said first transmission line as to establish a voltage maxima of saidthird harmonic frequency at said dee electrode, a first oscillatoradapted to excite said first transmission lines at said fundamentalfrequency, and a second oscillator adapted to excite said stubtransmission lines at said third harmonic frequency, said secondoscillator being keyed to said first oscillator in such a manner as tomaintain the proper phase and amplitude relationships to establish asubstantially square wave alternating potential between said deeelectrodes.

9. The combination of claim 5 wherein said second oscillator ischaracterized by providing an excitation having -a peak valuesubstantially one-ninth that of said first oscillator at said deeelectrodes.

10. In a radio frequency system for a cyclotron having a pair of deeelectrodes, the combination comprising two trans-mission line systemsrespectively connected to said dee electrodes, means coupled to saidsystems to provide an excitation voltage having a fundamental frequencycomponent and a third harmonic frequency component, and means includedin each of said systems to maximize both frequency components at saiddee electrodes to provide a substantially square wave voltage.

22, No. 2, The design of cyclotron oscillators, Backus, pages 84-92.

